The invention relates generally to video graphics processing and more particularly to a method and apparatus for adjusting bias and amplitude of a video signal.
Video signals originate from a variety of sources including TV tuners, digital video disc (DVD) players, video cassette recorders (VCRs), video cameras, and the like. In many situations, the video signals are in an analog format. In order to allow these analog video signals to be processed using digital processors to create special effects, merge signals, or perform other digital signal processing, the analog signals must be converted to a digital format.
In many cases, the analog-to-digital (A/D) converters used to perform the conversion of these video signals have stringent input requirements. In order to allow for conversion, the video signals may have to be adjusted such that input requirements of the A/D converters are met. The conditioning required to place these video signals within the specifications of the A/D converters may include adjusting the amplitude and the DC bias of the signals.
In many cases, the amplifiers utilized to adjust the amplitude of the video signals are limited by the power supplies of the amplification circuit. This can cause portions of the signal to be clipped, or truncated. This clipping or truncation can result in a degraded video signal that is undesirable in many applications.
Another problem that often arises in preparing the video signal for A/D conversion is the susceptibility of the signal to noise within the circuit. This is especially true in integrated circuit implementations that include a large amount of digital circuitry. The switching of a large amount of digital gates in close time proximity can cause spikes on the power supplies of such integrated circuits, thus injecting noise into the video signal. This noise can result in a degraded video signal.
Video signals from various sources can expect different termination resistance values. For example, the output of a DVD player may incorporate some level of termination resistance that differs from that of a VCR. As such, the termination resistance provided on the circuit that performs the A/D conversion may have differing effects on received video signals. Some video signals may be greatly attenuated, whereas others may have more amplitude than is desired. The ability to adjust the amplitude of such signals prior to conversion is important, and as stated earlier, should be accomplished without inducing truncation or clipping of the signals.
Therefore, a need exists for a method and apparatus for preparing video signals for analog-to-digital conversion that reduces susceptibility of the video signals to noise and allows for amplitude adjustment of the video signal without the truncation that can occur due to power supply limitations.